Apt 拒绝从 Debian backports 安装内核固件包

为了让我的 Ryzen 集成 GPU 正常工作，并最终尝试将屏幕分辨率设置为 1024x768 以上，我的理解是我至少需要 Linux 4.15。

我正在使用 4.9 上的 Debian Stretch。所以我听说了“backports”（https://backports.debian.org/Instructions/）并被告知添加以下行：

deb http://ftp.debian.org/debian stretch-backports main

到 /etc/apt/sources.list。我已经这样做了，并且我已经运行了apt-get update.

这使得 4.17 内核映像 (linux-image-4.17.0-0.bpo.3-amd64) 在包管理器中可用，我很高兴地安装了它，内核启动并运行良好。

下一条错误消息是：

[drm:amdgpu_pci_probe [amdgpu]] *ERROR* amdgpu requires firmware installed
See https://wiki.debian.org/Firmware for information about missing firmware

好的.....所以我也需要将内核固件包升级到向后移植的版本。

我在这里卡住了。

我在 Synaptic 中看不到任何固件包的任何反向移植版本。Debian backports 说明页面建议使用-tapt-get 的 (target?) 参数来选择 backports 存储库或其他任何东西，但这并没有带来任何成功。

这是我正在运行的命令：

root@debian:~# apt-get -t stretch-backports install firmware-linux
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package firmware-linux is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  firmware-misc-nonfree

E: Package 'firmware-linux' has no installation candidate

root@debian:~# apt-get -t stretch-backports install firmware-linux-nonfree
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package firmware-linux-nonfree is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  firmware-misc-nonfree

E: Package 'firmware-linux-nonfree' has no installation candidate

root@debian:~# apt-get -t stretch-backports install firmware-misc-nonfree
Reading package lists... Done
Building dependency tree
Reading state information... Done
firmware-misc-nonfree is already the newest version (20161130-3)
0 upgraded, 0 newly installed, 0 to remove and 212 not upgraded

root@debian:~# apt-get -t stretch-backports install firmware-linux-free
Reading package lists... Done
Building dependency tree
Reading state information... Done
firmware-linux-free is already the newest version (3.4)
0 upgraded, 0 newly installed, 0 to remove and 212 not upgraded

我不知道我要安装的哪个软件包是我真正想要的，但关键是没有看到它们中的任何一个。但我知道它们存在，因为它们在网上列出：https ://packages.debian.org/stretch-backports/firmware-linux （版本：20180518-1~bpo9+1）。

从昨晚开始，我一直在努力说服 apt 承认这一事实，但没有成功。

我的 sources.list 中的行是：

deb http://security.debian.org/debian-security/ stretch/updates main
deb-src http://security.debian.org/debian-security/ stretch/updates main
deb http://ftp.debian.org/debian/ stretch-backports main